Sapphire wafers are an important semiconductor substrate. They are especially important for the development of gallium nitride based materials technology, which is used in blue spectrum light emitting diodes (LEDs). The production of high brightness LEDs in the blue spectrum is a relatively recent optoelectronics technology. The demand for nitride based LEDs, such as bright blue, bright green and other color LEDs, currently exceeds the industry's capability to supply them. Sapphire based device separation, however, remains a significant obstacle to efficient fabrication. Current separation techniques waste valuable wafer surface area, involve costly consumables, and have long process times.
Semiconductor fabrication processes involve fabricating several thousand individual devices, or dies, on one wafer. After processing and testing, the wafer may be thinned and the dies must be separated from the wafer. Separation has been traditionally accomplished using either a dicing saw or a scribe-and-break process, both of which rely on diamond chips to cut the material. These two processes have been very effective on silicon and III-V substrates, because the material is much softer than diamond. However, sapphire's crystal structure, crystal orientation, inherent hardness, and material strength inhibit these methods from working well.
Specifically, the diamond's edge dulls quickly when applied to sapphire. To compensate, dicing saw blades designed to cut sapphire contain diamonds in a resin matrix. The dicing blades wear quickly to constantly expose new, sharp diamonds. Although processing times and the number of blades is dependent on die size, studies have shown that completely dicing a 17 mil thick sapphire substrate into 16 mil×16 mil die would require up to four blades and over 2 hours of process time and the maximum yield would be 25%. A 4 mil thick sapphire substrate completely shatters during dicing. These low yields make it difficult to meet commercial demand. The yields are low because the minimum blade thickness is 8 mil. This results in a kerf width of >0.010″. Thinner blades, however, produce poor quality cuts. A significant amount of available device surface area is therefore wasted during wafer sawing.
The scribe-and-break separation process also relies on a sharp diamond edge or facet. The scribe tip has a diamond head which is quickly dulled by the sapphire. This requires frequent and costly tip replacement Due to these factors, a sapphire dicing process will produce too low a yield. Moreover, both the sawing and diamond scribing process become very complex due to diamond wear.
Another method for device separation is discussed in U.S. Pat. Nos. 5,151,389 and 5,214,261, both of which are issued to Zapella. These references discuss a method for dicing semiconductor substrates using an excimer laser beam. This method uses a laser beam that is oriented out of normal with respect to the substrate to ensure non-tapered cuts. A drawback of this method is that the substrate and the laser beams must be maintained within the critical out of normal ranges. A further drawback is that a polyimide coating is used to prevent “dust” from settling onto the surface. The removal of this coating via chemical peeling introduces the possibility of contamination.